1. Field of the Invention
The present invention relates to a stacked conformation radiotherapy system which irradiates a diseased part with a particle beam of carbon, neon or the like in three dimensions for the purpose of the therapy of a cancer, a malignant tumor or the like, and a particle beam therapy apparatus which employs the stacked conformation radiotherapy system.
2. Description of the Related Art
In order to irradiate a diseased part with a particle beam of appropriate dose for the purpose of the therapy of a cancer, a malignant tumor or the like, a conformability needs to be heightened by performing so-called “stacked conformation radiotherapy” in which the irradiation region of the particle beam is brought into agreement with the shape of the diseased part existing in three dimensions. To this end, it is important that the shape (leaf positions) of a multi-leaf collimator is appropriately set, and that both the spatial distributions of an irradiation dose in a horizontal direction (on the plane of an irradiation field) and an irradiation dose in a vertical direction (a depth direction) are homogenized to the utmost so as to apply a homogeneous dose to the whole diseased part.
Here, as a method for homogenizing the dose distributions of the planar irradiation field which irradiates the diseased part, there is a wobbling method (refer to, for example, Patent Documents 1, 2, 3 and 4 being JP-A-2006-288875, JP-A-2001-326098, JP-A-2000-331799 and JP-A-2005-103255, respectively). In the wobbling method, a pair of wobbler electromagnets are arrayed so that the directions of the magnetic fields thereof may become orthogonal to each other, and currents whose cycles are equal and whose phases differ 90 degrees are caused to flow through the respective wobbler electromagnets, thereby to magnetize the wobbler electromagnets. Thus, the particle beam emitted from an accelerator is turned and deflected in orthogonal directions within a plane perpendicular to its traveling direction by the magnetic fields of the wobbler electromagnets. As a result, as shown in FIG. 16, the particle beam depicts a circular revolving orbit S every fixed cycle Tw (hereinafter, a time period which is expended in depicting the revolving orbit S for one revolution shall be called the “wobbler cycle Tw”). On this occasion, when the scattering angle of scattering ascribable to a scatterer disposed midway of the irradiation path of the particle beam and the radius R of the revolving orbit S are optimally set, two particle beam distributions P1 and P2 opposing to each other on the revolving orbit S overlap as shown in FIG. 17, so that a dose distribution P0 in a radial direction becomes a flat distribution within a plane containing a revolving center O. Accordingly, when the particle beam is projected along on the revolving orbit S so as to depict a complete round for one revolution, the dose distribution on the plane of the irradiation field becomes homogeneous.
On the other hand, in order to homogenize the dose distribution in the vertical direction (depth direction), processing has heretofore been executed as shown in FIG. 18. More specifically, the irradiation energy of the particle beam is altered every layer in the depth direction by a range shifter, in correspondence with the size L of the diseased part in the depth direction. Thus, while extended Bragg peaks D1, D2, . . . are being moved along the depth direction, the irradiation dose is heightened more at the deeper layer position of the diseased part, and it is gradually lowered more as the depth becomes shallower. That is, while the extended Bragg peaks D1, D2, . . . are being moved along the depth direction of the diseased part, the irradiation dose is adjusted in accordance with the hierarchy of the irradiation, whereby the whole dose distribution Dt obtained by cumulating the extended Bragg peaks D1, D2, . . . of the respective layers becomes flat in correspondence with the size L of the diseased part in the depth direction.
In this case, since the irradiation dose of the particle beam is higher at the position of the deeper layer, the irradiation time period of the particle beam is proportionally longer if particle beam intensities at the respective layers are equal. At the deeper layer of the diseased part, therefore, the number of revolutions of the particle beam (hereinafter, termed the “number of wobbler revolutions”) becomes larger. In other words, as the hierarchy of the irradiation advances more (the irradiation region becomes shallower), the number of wobbler revolutions per a layer becomes smaller.
Meanwhile, in the prior-art stacked conformation radiotherapy as stated in Patent Documents 1-4, the timings of the emission and stop of the particle beam from the accelerator such as a synchrotron, that is, the irradiation period of the particle beam have/has been independent of the wobbler cycle Tw. More specifically, the magnetization signals of the pair of wobbler electromagnets are endowed with a fixed relationship so as to have the phase difference of 90 degrees from each other, but their wobbler cycle Tw is set independently of that irradiation period Tb of the particle beam which is time-divided in order to obtain a predetermined irradiation dose.
On the other hand, in the stacked conformation radiotherapy, the particle beam irradiation is ended when the irradiation dose set every layer beforehand is reached. On this occasion, in the case where the wobbler cycle Tw is set independently of the irradiation period Tb of the particle beam as stated above, the emission of the particle beam from the accelerator is stopped before the particle beam makes one revolution of the revolving orbit, and an unirradiated region which is not irradiated with the particle beam appears at part of the revolving orbit.
Especially, in the stacked conformation radiotherapy, the number of wobbler revolutions per layer becomes smaller gradually as the hierarchy of the irradiation advances more (the irradiation region becomes shallower), as stated before. Therefore, in the case where the irradiation with the particle beam has been stopped midway of the revolving orbit, a place where parts lacking the particle beam irradiations overlap several layers appears, and the dose distribution in the depth direction becomes drastically inhomogeneous.